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Black silicon formed by helium plasma irradiation has attractive surface morphologies in terms of solar cell fabrication, because it has good photon absorption property across the solar spectrum. It also does not need any masking process, which is sometimes employed in the fabrication of pyramid-like microstructures. Black silicon formed by helium plasma irradiation has potential not only for solar cell applications, but also in the fields of photonics, optics, and ultrasonic wave generation. Here, the nano-cone structure formation process, detailed observations of surface morphologies with TEM as well as SEM, the optimal He irradiation conditions, surface reflectivity over UV–visible-NIR wavelengths, and scattered light enhancement with Raman spectroscopy in addition to X-ray diffraction measurements were studied. Investigation of electronic properties is reported, including good resistivity of the Si black surface without an ITO electrode.

Fiber-form nanostructures grown on tungsten (W) surfaces with helium (He) plasma irradiation exhibit an excellent thermal radiative property at low sample temperatures up to 1200 °C. To maintain this radiative property at higher temperatures, the recovery to undefected surface and annealing effect of nanostructured W surfaces need to be investigated while focusing on the dopant effect. TFGR-W-1.1%TiC/H (toughing, fine-grained recrystallized tungsten with TiC dispersoids) was experimentally found to have the best characteristics among the tested doped W samples. However, it was noted and discussed that a further improvement at higher temperatures needs an additional surface treatment. Possibilities of maintaining complicated structures with nano-scale lengths are discussed in terms of dopant pinning effect and self-diffusion on tungsten surface.

Growth mechanisms of complex tungsten (W) fuzzy nanostructure on helium-plasma-irradiated W surface have been extensively studied since the discovery of the phenomenon in 2006. However, the growth process of complicated W nano-fibers from the atomistic and crystal structure standpoints is not well understood. In this study, the X-ray diffraction (XRD) and transmission electron microscopy (TEM) were performed to investigate complexity of nano-fibers as well as their crystallinity. Small-angle-incident XRD on the substrate with fuzz and He-defected bulk surface was used to determine grain distribution among contributions of different crystal orientation families, showing the difference in grain distribution of surface fuzz layer and the basing W bulk surface with helium defects compared with the distribution of pristine powder metallurgy W (PM-W). Peak shifts and broadenings of X-ray diffracted spectra with respect to pristine PM-W substrate and TEM observations were discussed in terms of crystallinity and He-bubbles induced strain. It is speculated that the grain distribution of the He-defected W with nano-fibers would be very similar to that of bulk α-phase W globally. Moreover, the scraped-off fuzz aggregate without basing plate makes it possible to conduct XRD using a thin glass capillary tube. The grain distribution of pure fuzz aggregate was found to be very similar to natural α-phase W with intensification of high grain orientation modes. The process of such high modes may bring morphological complexity to nano-fibers because grain boundaries play a role of turning surfaces along nano-fibers.

The radiative cooling capability of tungsten with a fiber-form nanostructure is evaluated precisely using an electron-beam power system. It is found that the total emissivity of black nanostructured tungsten is very close to 1.0, meaning an almost complete black body. The discrepancies in the determination of the total emissivity between the electron-beam method and a fixed biasing procedure are discussed in terms of the power transmission through the plasma sheath, focusing on the ion energy reflection and cooling associated with electron emission.

The initial stage of fiber-form nanostructure growth on metal surface with helium plasma irradiation is illustrated, taking recent research knowledge using a flux gradient technique, and including loop-like nano-scale structure as precursors. The growth mechanism of fibers is discussed in terms of the shear modulus of various materials that is influenced by the helium content as well as the surface temperature, and the mobility of helium atoms, clusters and/or nano-bubbles in the bulk, loops and fibers. This model may explain the reason why tantalum does not provide fiber-form nanostructure although the loop-like structure was identified. The model also suggests the mechanism of an existence of two kinds of nanostructure of titanium depending on surface temperature. Industrial applications of such nanostructures are suggested in the properties and the possibilities of its growth on other basic materials.

The generation of tungsten fuzzy nanostructure by exposure to helium plasma is one of the important problems for the use of tungsten material as divertor plates in nuclear fusion reactors. In the present paper, the formation mechanisms of the helium bubble and the tungsten fuzzy nanostructure were investigated by using several simulation methods. We proposed the four-step process which is composed of penetration step, diffusion and agglomeration step, helium bubble growth step, and fuzzy nanostructure formation step. As the fourth step, the formation of the tungsten fuzzy nanostructure was successfully reproduced by newly developed hybrid simulation combining between molecular dynamics and Monte-Carlo method. The formation mechanism of tungsten fuzzy nanostructure observed by the hybrid simulation is that concavity and convexity of the surface are enhanced by the bursting of helium bubbles in the region around the concavity.

Power transmission factor (PTF) through the sheath of deuterium plasmas with hot electron component on virgin as well as nanostructured tungsten due to helium pre-irradiations are studied experimentally and compared with a PTF theory. PTF for nanostructured tungsten is larger than that for normal one, the reasons of which are discussed in terms of surface characteristics.

Power transmission factors (PTF) through the sheath of helium plasmas with hot electron component against two kinds of tungsten wall, virgin as well as nanostructured one due to helium irradiation are studied theoretically and experimentally. In the present theory the ion-induced electron emission in addition to secondary electron emission is taken into account, contributing to the cooling of the wall. Some discussions on the comparison between the measurement and the modified PTF theory are given. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

When tungsten is exposed to helium plasmas, nanostructures are grown on the surface. Field emission properties of the tungsten samples are measured; it is revealed that the field emission was significantly increased as changing the surface morphology by the plasma irradiation. Demonstration of arcing and unipolar arcing on the nanostructured tungsten was conducted in the linear plasma device NAGDIS-II, which simulates the fusion relevant conditions. Arcing can be easily initiated in response to a transient heat load with a pulsed laser, which demonstrates transient heat loads in fusion devices. It is thought that the increase in the field emission and the decrease in the thermal diffusivity near the surface lead to the initiation of arcing. The behavior of the free running arc spots are observed with fast framing camera. Initial behavior of initiated arc spots and an interesting behavior, bifurcation, random motion, etc. are shown.

The physical properties of tungsten with nanostructure on its surface are investigated focusing on the thermal radiation and cooling characteristics. First, direct evidence of substantial W surface cooling has been clearly shown with use of a very thin thermocouple inserted into W target, which solves an uncertainty associated with a radiation thermometer. Second, the above measurements of W surface temperature make it possible to estimate quantitatively the total emissivity from which we may evaluate the radiative power through the Stefan–Boltzmann equation, which is very important for mitigation evaluation of a serious plasma heat load to the plasma-facing component.

One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused by helium plasma irradiation since helium is a fusion product. The fiber-formed nanostructure is thought to have a possible weakness against the plasma heat flux on the plasma-facing component and also may destroy the reflectivity of optical mirrors. In this paper an interesting method for the recovery of such tungsten surfaces is shown. The recovery process depends on the grade and manufacturing process of tungsten materials.

A unique plasma-generation device called the “Aichi Institute of Technology-Plasma Irradiation Device” has been developed for plasma–wall interaction (PWI) studies toward realization of a fusion reactor. It has power-saving characteristics as well as compactness due to the employment of a multicusp magnetic field configuration instead of a strong axial magnetic field. Helium as well as argon plasmas are generated, showing an outstanding property of production of the hot-electron component which can be controlled by changing the working gas pressure. Utilizing such outstanding properties, some typical studies on PWI are introduced. An azimuthally asymmetric confinement depending on the direction of magnetic field lines, particularly for energetic electrons, has been found. Some discussions on the physical mechanisms are given. IEEJ Trans 2011 DOI: 10.1002/tee.21801

Helium plasma irradiation and electron heating experiments were conducted using tungsten in the divertor simulator NAGDIS-II. Helium plasma irradiation to tungsten led to the formation of nanostructures on the surface, while the nanostructures were annihilated after the potential of the specimen was changed to positive for several 10 min so that electrons irradiated the sample without ion irradiation. The specimens were analyzed in detail by transmission electron microscope with the help of focused ion beam technique. It is revealed that the helium nano-bubbles still remained even after the nanostructures were disappeared from the surface. Porosity of the nanostructured tungsten was measured from the TEM images.

Behaviors of an arc spot ignited on a tungsten electrode in response to a transient heat load are investigated in the linear divertor simulator NAGDIS-II using a pulsed laser as a transient heat source. Tungsten neutral emission from the arc spot is observed with an image intensified fast framing camera. When arcing is initiated, the tungsten emission continues much longer than the laser pulse width of ∼0.6 ms. The ignition of arcing becomes much easier compared to clean surface when the tungsten surface is exposed to the helium plasma and morphology is slightly changed.

The effects of temperature excursion on the helium defects of tungsten surface have been investigated by using compact plasma device AIT-PID (Aichi Institute of Technology – Plasma Irradiation Device). An initial stage of bubble formation has been identified with an order of smaller (sub-micron) bubbles and holes than those in the past in which the micron size is the standard magnitude. The radiation cooling has been detected when a blacking of tungsten surface coming from nanostructure formation is proceeding due to an increase in the emissivity. The temperature increase to the domain (∼1600 K) in bubble/hole formation from that in nanostructure formation has been found to bring a constriction in diameter and a reduction in length of fiber-form nanostructure.

In this article, we have analyzed the measured probe characteristics of microwave-sustained Ar and He plasma jets at atmospheric pressure using the high-pressure probe theory developed by Talukder et al. in order to determine the generated plasma parameters (electron temperature, Te, electron density, Ne, plasma potential, ϕp). The calculation shows some unusual temperature profiles (both axial and radial) as well as provides some over estimation of electron temperature measured from the distorted I-V characteristic due to a smaller electron to ion saturation current ratio coming from the limited electron saturation current. In order to overcome these limitations, the measured probe characteristics are also analyzed by using the asymmetric double probe theory developed by our group, which provides reasonable value of plasma parameters. The influence of microwave power and gas flow rate on the plasma parameters are also investigated (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This overview describes recent studies of nuclear fusion by using divertor plasma simulators generating high heat flux plasma. The experimental results regarding plasma-material interaction related to tungsten and carbon dusts, and detached recombining plasma are reviewed. © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Oxy-combustion generates a high-temperature field of more than 3000 K and represents a nitrogen-free process. In order to apply oxy-combustion to next-generation power plants and high-temperature industrial technology this process has been enhanced by a new method, referred to as ‘plasma-assisted combustion’. This method uses to control the oxy-combustion flame with the radio-frequency electromagnetic force acting on the hightemperature field of the oxy-combustion flame. In order to establish the plasma-assisted combustion technique it is necessary to assess the influence of the electromagnetic field on the flame parameters, i.e. to compare the main plasma properties (conductivity, electron density and temperature) of the oxy-combustion flame with and without the assistance of the electromagnetic field. For that purpose we have developed a reciprocating double probe method to measure the plasma properties of the oxy-combustion flame. In this paper we present (i) the experimental part (ii) the model that enables to estimate the electron temperature and density from the conductivity derived from the double probe measurements and (iii) measurements without the applied electromagnetic force. At the centre of the flame, the measured electrical conductivity is (1.3 ± 0.7) × 10–4 S/m which gives an electron temperature of 2800 ± 100 K and an electron density of (1.2 ± 0.5) × 1015 m–3. The electron temperature in oxy-combustion is equivalent to the adiabatic flame temperature. The equilibrium adiabatic flame temperature in the oxy-combustion of methane is estimated to be approximately 3100 K, which is in good agreement with the electron temperature measured in this study. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulations of the helium radiative properties carried out with the recently developed multi-level meta-stable resolved collisional-radiative code SOPHIA discovered new unique emission lines to analyze optically thick divertor plasmas relevant for ITER. The comparison of their time dependent line emission obtained from the NAGDIS-II plasma simulator experiments with time dependent temperature probe measurements shows a strong correlation. This indicates that line intensity fluctuations can be transformed to the important quantities of density and temperature fluctuations. A transformation method based on integral line intensity ratios which can be recorded with high time resolution is discussed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamuraet al2002Plasma Sources Sci. Technol.11A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

We have investigated the influence of low energy helium and hydrogen/deuterium plasma irradiation to tungsten or tungsten coated graphite samples. The experimental conditions of helium hole/bubble formation and hydrogen blister formation were summarized. The tungsten coated graphite exposed to a helium plasma shows different surface modification from those of bulk tungsten. We also demonstrated a transient heat load onto a damaged tungsten surface due to a helium plasma exposure by a laser-beam irradiation. Simultaneous helium plasma and laser pulse irradiations lead to a further roughing of the tungsten surface.

Toroidal simulator for boundary plasma studies may have a substantial potential in plasma and impurity transport under an open magnetic field configuration and plasma-surface interactions under a grazing incident magnetic field line to the wall of the plasma-facing component in fusion devices. NAGDIS-T is the toroidal version in NAGDIS-series plasma simulator with RF Joule heating for toroidal plasma generation. A simple numerical analysis was employed to predict the plasma parameters in this new machine, considering the particle and energy balance equations. It was found that the gas pressure dependence of the plasma density is very similar to that obtained in the preliminary experiment with moderate RF power. The analysis predicts a plasma density exceeding 10 19 m -3 by injecting the RF power of more than 10 kW which will be available soon.

Low-energy deuterium (D) plasma exposure on tungsten (W), which is an important material for ITER, results in blister formation on the surface. Blister formation increases both micron-sized dust production and D retention. Blister formation depends greatly on surface pre-treatment. Deuterium plasma exposure on mirror-finished powder metallurgy W at 500 K for 3 h forms a blister with a diameter of a few hundred micrometres on the surface. Blister formations on the mechanically-polished and helium-pre-exposed surfaces are drastically suppressed. Deuterium retention is also reduced on both the mechanically-polished surface and the helium-pre-exposed surface compared with that on the mirror-finished surface. The suppressive effect of blister formation on the mechanically-polished surface is maintained for 50 h. The size of blisters and D retention on mirror-finished surface exposed for 50 h increases by some degrees (500 µm, 7 × 1020 m−2) compared with that on mirror-finished surface exposed for 3 h (200 µm, 5 × 1020 m−2), but is not proportional to the exposure time. The saturation level of D retention even on blister-rich surfaces seems to be lower than an order of 1021 D2 m−2.

Experimental studies on mesoscale erosion of tungsten surface due to low-energy and high-flux helium/deuterium exposure are systematically described. Following the historical description of the accidental finding of helium bubbles and holes, possible changes in tungsten properties like the A coefficient and the work function of the rmo electron emission, the emissivity and the energy reflection coefficient are discussed in terms of thermal bifurcation. Multi-dimensional parameter area for He bubble formation is identified in the space of tungsten temperature, ion fluence and incident ion energy. In particular, the threshold energy was surprisingly found to be as low as 6 eV, in association with which the formation mechanism is proposed. Finally, the effect of bubbles and holes formation on deuterium retention is quantitatively clarified.

Surface modification of polycrystal tungsten due to low-energy and high-flux helium plasma exposure was investigated. Micron-sized holes and bubbles were formed on the surface at a surface temperature of 2200 K even when the incident ion energy of helium was 10 eV. Hole formation was significantly reduced on the surface when the incident ion energy of helium was around 5 eV and no surface modification was seen at less than 5 eV. Coalescence of micron-sized helium bubbles was observed in the cross section of a W sample.

A radio frequency (rf) inductive discharge in atmospheric pressure range requires high voltage in the initial startup phase and high power during the steady state sustainment phase. It is, therefore, necessary to inject high rf power into the plasma ensuring the maximum use of the power source, especially where the rf power is limited. In order to inject the maximum possible rf power into the plasma with a moderate rf power source of few kilowatts range, we employ the immittance conversion topology by converting a constant voltage source into a constant current source to generate efficient rf discharge by inductively coupled plasma (ICP) technique at a gas pressure with up to one atmosphere in argon. A novel T-LCL immittance circuit is designed for constant-current high-power operation, which is practically very important in the high-frequency range, to provide high effective rf power to the plasma. The immittance conversion system combines the static induction transistor (SIT)-based radio frequency (rf) high-power inverter circuit and the immittance conversion elements including the rf induction coil. The basic properties of the immittance circuit are studied by numerical analysis and verified the results by experimental measurements with the inductive plasma as a load at a relatively high rf power of about 4 kW. The performances of the immittance circuit are also evaluated and compared with that of the conventional series resonance circuit in high-pressure induction plasma generation. The experimental results reveal that the immittance conversion circuit confirms injecting higher effective rf power into the plasma as much as three times than that of the series resonance circuit under the same operating conditions and same dc supply voltage to the inverter, thereby enhancing the plasma heating efficiency to generate efficient rf inductive discharges.

Low-energy and high-flux plasma exposures on tungsten surfaces result in blister/hole formation. Subsequent He plasma exposure at 2,000 K on the blister-rich surface forms a hole structure on the surface, including the blister surface, and some blisters collapse due to recrystallization and bubble formation on the blister skin. Grain ejections were observed after deuterium plasma exposure at 550 K on a W surface on which a hole structure had formed due to He pre-exposure at 1,600 K.

Generation and sustainment of rf induction thermal plasmas in atmospheric pressure range require a high voltage in the initial startup phase and a high rf power during the steady state sustainment phase. The high rf electrostatic filed induced between induction coil conductor wound around a glass discharge tube generates surface electrostatic discharges in which the discharge current flows through the dielectric glass tube. After several hundreds microseconds the electrostatic discharges grow sufficiently to transfer to a volumetric electromagnetic discharge which is maintained by the inductive electric field. During this discharge mode transition, the loading impedance of the induction coil changes significantly. This loading impedance change of the induction coil leads a deviation from a series resonance condition of the LCR output circuit including the induction coil, which strongly affects the output characteristics of a semiconductor rf inverter power supply. In this paper transient response of the induction coil impedance in the initial startup phase of the rf thermal plasma generation is studied experimentally. Dynamic interactions of the rf inverter power supply with generated rf plasmas are also investigated and strong deterioration of the inverter output efficiency is found experimentally. Preliminary frequency tracking experiments show a successful improvement of the inverter output efficiency and absorbed rf power in the generated plasma.

We have derived an exact theoretical expression for the space-charge limited current from the solid surfaces adjacent to plasmas that is applicable for an arbitrary sheath voltage. Our expression shows that the space-charge limited current tends to saturate with the sheath voltage. This new formula is evaluated by 1-D Particle in Cell (PIC) simulation and experiment, and is in a good agreement with the simulation and experimental results. We have also obtained an analytical equation fitted to the new formula based on conventional Child-Langmuir formula by taking into account a more sophisticated dependence of the electrode potential and the plasma density through the effect of Debye shielding and a sheath expansion due to increased voltage across the sheath.

This paper will report on the proof-of-principle (POP) experiments conducted to demonstrate reduced wall recycling, using a laboratory-scale test unit, constructed based on the concept of moving-surface plasma-facing component (MS-PFC). In this concept, the moving-surface exposed to edge plasmas in steady state magnetic fusion devices is continuously deposited ex-situ with a getter material, so that particle trapping capabilities can be regenerated prior to the subsequent exposure. In our previous paper, the construction details of the MS-PFC test unit and the first results in the case of titanium gettering was reported, but in the present paper preliminary results in the case of lithium gettering will be presented for comparison. Results indicate that the H α light intensity used as the measure of hydrogen recycling is reduced by ∼6% due to titanium gettering and by ∼12% due to lithium gettering, both at steady state.

Penetration processes of rotating helical magnetic perturbation (RHMP) into tokamak plasmas have been studied on a small tokamak device HYBTOK-II for dynamic ergodic divertor experiment preparing in TEXTOR. The radial profiles of RHMP in the plasma have been measured by inserting small magnetic probes into the plasma. The penetration of RHMP has been discussed by using the observed radial profile of RHMP in the plasma based on the growth of magnetic islands (i.e. tearing mode). In the case of the low Doppler-shifted frequency observed from the plasma, the amplification of RHMP in the plasma due to a spatial modification of plasma current has been found. A large Doppler-shifted frequency produces an expected attenuation of RHMP near the magnetic resonance surface due to screening current flow. A decrease of the amplification of RHMP originated from the suppression of the growth of magnetic islands due to a rapid spatial scan of magnetic resonance layer has been found by plasma current oscillation.

In the magnetic fusion community much attention has recently been directed to particle control in the existing confinement experiments, as it is recognized that the core plasma performance can directly be affected by the edge conditions. However, it is still true that there is no clear strategy to cope with the needs for steady-state reactor operation. That is that, due to the saturation nature, wall conditioning such as boronization is not applicable for fuel and impurity particles control in steady-state devices, requiring a start from enabling concepts development. As a possible solution, the concept of moving-surface plasma-facing components (MS-PFCs) was proposed in our previous work and in the present work proof-of-principle experiments have been conducted. In this concept, the plasma-facing surface is mechanically circulated out-of-pile for regeneration of particle capturing capabilities by coating getter films on it. A prototypical MS-PFC test unit has been constructed for these proof-of-principle experiments, employing a rotating copper drum as the moving surface and titanium as the getter material. It has been indicated that, from Hα light intensity data taken in front of the rotating target, relative to no getter cases, approximately 6% reduced hydrogen recycling has been achieved at steady state. In support of these data, a first-order particle balance model predicts a 7% reduction in hydrogen recycling.

This study presents experimental results on hydrogen blister formation on powder metallurgy tungsten (PM-W) surface under low energy ( 1021 m−2 s−1) hydrogen plasma irradiation in a divertor plasma simulator-NAGDIS-I. The tungsten samples were exposed to steady-state hydrogen plasma at various sample temperatures and fluences. Hydrogen blister formations are clearly observed on tungsten surface at the surface temperature below 950 K. The blister size is from a few 10 μm to a few 100 μm. It was found that the blister formations obviously depend on the surface temperature and the incident hydrogen ion fluence. No blisters are observed on tungsten surface at the surface temperature above 950 K even if the fluence is high enough.

We achieved a potential well more than fifty times deeper than the plasma electron temperature, down to -0.3 kV with the electrode biasing, by the cross-field radial arcing current of up to 150 A which flows between the small electron-emitting electrode (LaB6) inserted to the plasma center and the stainless-steel wall of vacuum chamber. The empirical scaling of the radial resistance of plasma Rr on the radial arcing current Ib and the toroidal magnetic field BT is found to be Rr ∼ Ib-1.2BT1.0. Structural dynamics of negative potential well formation is observed: the potential dip propagates from the plasma core to the periphery. The ion saturation current also increases, starting from the inner region. The fluctuation frequency in Isat changes abruptly from 200 kHz to 30 kHz in a time of around 0.25 ms after negative biasing.

Recombination in a helium and helium-hydrogen plasma was experimentally investigated in the linear divertor simulator NAGDIS-II. To identify the physical mechanisms, electron-ion recombination (EIR) and molecular assisted recombination (MAR), the plasma parameters in the recombining region have been measured. We used spectroscopic techniques on the basis of continuum emission and line emission from highly excited levels due to EIR and Langmuir probes. The dependence of the plasma parameters on the neutral pressure in a pure helium plasma and in a helium-hydrogen mixture shows clearly that recombination is caused by different processes in the two cases. In a pure helium plasma a roll-over and a following decrease of the electron density as well as a monotonic decrease of the electron temperature down to 0.1 eV were observed when increasing the helium pressure. Here EIR is the recombination mechanism. Conversely, when hydrogen is added to the helium plasma, the electron density decreases monotonically while the central electron temperature remains above 1 eV. This particular behaviour in the helium-hydrogen plasma can only be explained by the dominant action of MAR.

Influence of energetic electrons on the light emission from detached recombining plasmas has been investigated by using Collisional-Radiative model, where the rate coefficients were modified by taking electron beam components as the energetic electrons into account. Line emission intensities are changed, strongly depending on the electron beam energy, because the population density distribution is determined by not only recombination processes but also electron impact excitation processes due to the electron beam component such as re-excitation, excitation from meta-stable states and excitation from the ground state. The influence of energetic electrons on the inverse ELMs was also discussed. Furthermore, it is found that bulk electron temperature evaluated with the method of Boltzmann plot tends to decrease dramatically with an increase in the electron beam density.

A Monte Carlo impurity transport code based on object-oriented method has been developed. Edge plasma in fusion devices has a lot of impurity atoms and molecules such as hydrocarbon species, C x H y due to low electron temperature in the edge. Various atomic/molecular processes have been considered in the edge plasma region and we need to determine which process is essential in edge plasma conditions. To take such complicated processes into account in a systematic way, Monte Carlo impurity simulation, which has a strong flexibility in dealing with various atomic and molecular processes, would be useful. By employing the object-oriented method in the simulation code, we can handle arbitrary atomic and molecular processes.

Microwrinkle structures with a pitch of less than 100 nm and up to 600 nm on refractory metals like tungsten (W) and molybdenum, which are irradiated by noble gas ions such as neon and helium, have been studied systematically. The wrinkle formation mechanism is thought to be a buckling of the hard surface layer supported by the soft elastic substrate, which is induced by a penetration of noble gas species from the irradiated surface. Microwrinkle forms on this structure under lateral compressive strain/stress fields coming from thermal constriction on the way to substrate cooling. Such a process should be anticipated when walls in a fusion reactor are attacked by heat pulses such as edge localized modes and/or vertical displacement events, and therefore might be an initial stage of W surface damage.

In order to find the causes of the strong anomaly of current-voltage characteristics of Langmuir probe observed in detached recombining plasmas in a linear divertor plasma simulator, NAGDIS-II, we have investigated plasma resistance along a magnetic field and potential fluctuations in the detached recombining plasmas. Simple calculation on the ratio between the plasma length, at which plasma resistance and resistance of ion sheath formed around a probe tip become equal, and an electron collection length indicates that the evaluation of electron temperature T e becomes inaccurate at T e of less than 0.6 eV when plasma density and neutral pressure are 1.0 x 10 18 m -3 and 10 mtorr, respectively. The potential fluctuation in detached recombining plasmas was found to be so large compared to T e /e, which can also modify the probe characteristics.

A comprehensive investigation has been performed of the static and dynamic behaviour of detached recombining plasmas in the linear divertor plasma simulator NAGDIS-II. For stationary plasma detachment, the transition from electron-ion recombination (EIR) to molecular activated recombination (MAR) has been observed by injecting hydrogen gas into high density helium plasmas. The particle loss rate due to MAR is found to be comparable to that of EIR. Experiments have also been performed by the injection of a plasma heat pulse produced by RF heating into the detached helium plasma to demonstrate the dynamic behaviour of volumetric plasma recombination. Negative spikes in the Balmer series line emission were observed and found to be similar to the so called negative ELM observed in tokamak divertors. Observed Balmer spectra were analysed in detail using the collisional-radiative model. A rapid increase of the ion flux to the target plate was observed associated with the re-ionization of the highly excited atoms generated by EIR.

Effects of condensible plasma impurities on the material erosion behaviors have been investigated by improving the previous zero-dimensional model. The new model can describe more precisely the transition from the enhanced erosion due to impurities to reduced erosion with the formation of the impurity film. Model predictions have been proven by the experimental observations in NAGDIS-I. A water-cooled molybdenum target plate was bombarded with argon plasmas into which methane was introduced as carbon impurity. As the flow rate of the methane gas was increased, MoI emission line intensity, corresponding to sputtered particle flux, has been observed to decrease to a great deal.

A full continuous AC tokamak operation with a plasma current of ±600 A and a frequency of 60–100 Hz has been achieved for 1 min in CSTN-AC tokamak device by employing a new invertor power supply system. As an AC toroidal plasma source, it has been used for the studies of deposition of thin films. The duty of AC discharges can be controlled arbitrarily from less than 1 to 100%. A good titanium nitride(TiN) thin-film formation by reactive sputtering is obtained in high-duty AC tokamak with strongly ionized plasmas with a high-charged particle density beyond 10 18 m −3 , in an extremely low gas pressure of the order of 10 −5 mbar. The deposited thin films were analyzed by various surface diagnostics (SEM, EDAX, XPS, RBS).

High-heat-flux plasmas are produced by high-power (∼14 kW) ICRF heating using inverter power supplies in the linear diverter simulator NAGDIS-II. The power flow of radiated RF power is investigated by a calorimetric method. From conventional power calculation using antenna voltage and current, about 70% of the RF power is radiated into the plasma. But the increase of the plasma heat load along the magnetic field by RF heating is about 10% of the RF power. Through this experiment, we find that about half of the RF power is lost at the antenna surface through the formation of RF-induced sheath, and about 30% of the power is lost into the wall of the vacuum vessel through the charge exchange and elastic collision of ions with neutrals. © 2002 Scripta Technica, Electr Eng Jpn, 138(4): 34–41, 2002; DOI 10.1002/eej.1136

Molecular activated recombination (MAR) has been clearly observed for the first time in a divertor plasma simulator. A small amount of hydrogen gas puffing into a helium plasma strongly reduced the ion particle flux along the magnetic field, although the conventional radiative and three-body recombination processes were quenched. Careful comparison of the observed helium Balmer spectra with collisional radiative atomic and molecular data indicates that the population distribution over the atomic levels with relatively low principal quantum numbers can be well explained by taking the MAR effects into account. [S0031-9007(98)06573-9]